To further illustrate brilliant [Jan's answer][1], here are the experimental crystallographic data for both structures; also notice that reduced form contains $\ce{[Co(CN)5O]}$ units tilted at about $45^\circ$ dihedral angle, whereas in oxidized form those are perfectly aligned:

 1. Oxidized form $\ce{[(CN)5CoO2Co(CN)5]^6-}$, $d(\ce{O-O}) = \pu{1.447 Å}$, ICSD#6091 \[1\]:  
$\color{#909090}{\Large\bullet}~\ce{C}$;
$\color{#3050F8}{\Large\bullet}~\ce{N}$;
$\color{#FF0D0D}{\Large\bullet}~\ce{O}$;
$\color{#F090A0}{\Large\bullet}~\ce{Co}$.
[![enter image description here][2]][2]

 2. Reduced form $\ce{[(CN)5CoO2Co(CN)5]^5-}$, $d(\ce{O-O}) = \pu{1.243 Å}$, ICSD#4093 \[2\]:   
$\color{#909090}{\Large\bullet}~\ce{C}$;
$\color{#3050F8}{\Large\bullet}~\ce{N}$;
$\color{#FF0D0D}{\Large\bullet}~\ce{O}$;
$\color{#F090A0}{\Large\bullet}~\ce{Co}$.
[![enter image description here][3]][3]

### Bibliography

 1. Fronczek, F. R.; Schaefer, W. P. *Inorganica Chimica Acta* **1974**, 9 (Supplement C), 143–151 DOI: [10.1016/S0020-1693(00)89896-5][4].  
 2. Fronczek, F. R.; Schaefer, W. P.; Marsh, R. E. *Inorg. Chem.* **1975**, 14 (3), 611–617 DOI: [10.1021/ic50145a035][5].


  [1]: https://chemistry.stackexchange.com/a/82618/41328
  [2]: https://i.sstatic.net/xoyN3.png
  [3]: https://i.sstatic.net/SvhAx.png
  [4]: http://www.sciencedirect.com/science/article/pii/S0020169300898965?via%3Dihub
  [5]: http://pubs.acs.org/doi/abs/10.1021/ic50145a035